Tracked vehicle drive systems have a different arrangement of drive elements than wheeled vehicle drive systems. A tracked system typically consists of a track, one or more idler wheels, and a drive wheel. The drive wheel is coupled to a drive axle extending from the vehicle, and is also coupled to, and drives, a track. The drive wheel may have "teeth" or other elements on its outer surface to engage and positively drive the track. Alternatively, the drive wheel may engage the track strictly with frictional contact. To provide sufficient frictional engagement, the inner surface of the track is in contact with the drive wheel and the outer surface of the drive wheel usually has an outer polymeric surface, such as rubber. It is critical to design a system that provides sufficient friction between the drive wheel and the track. One method of accomplishing this is to provide a drive wheel at one end of the track and an idler wheel at another end of the track. Such an embodiment is shown in U.S. Pat. No. 5,312,176. A major drawback of this is the size of the components used. As can be seen in the '176 patent, the drive wheel has over 180 degrees of its surface in contact with the track, and thus has sufficient frictional engagement. However, due to the ground clearance necessary for a work vehicle, the drive axle must be elevated and consequently, the entire drive wheel itself is quite large. This design, due to the large drive wheel, prevents a pair of tracks from being installed on the same side of the vehicle (one towards the front and one towards the rear of the vehicle). Without two tracks on the same side of the vehicle, a four track articulated vehicle cannot be constructed. An alternative suspension configuration employs a smaller drive wheel located above and between two idler wheels, such as the wheel arrangement shown generally in FIG. 1. This arrangement, however, reduces the angular contact of the track about the periphery of the drive wheel, and thus reduces the frictional engagement area between the drive wheel and the track. Consequently, this wheel and track arrangement is used most commonly for tracked suspensions in which the drive wheel is mechanically interlocked with the track, and not for suspensions in which the drive wheel D is frictionally engaged to the track T. To provide more frictional contact and thus gripping force between track T and drive wheel D, drive wheel D can be elevated as shown in FIG. 1 in dished lines. The angle phi of contact of the drive wheel D with respect to the track is clearly greater when drive wheel D is elevated. Moving the drive wheel upward, however, requires that one move drive wheel D's axle upward. If a standard differential with a planetary output stage is employed, this also requires that the differential be moved upward. The net effect is to elevate the center of mass of the vehicle, which makes it more unstable.
An apparatus is needed that will provide for more effective frictional coupling of the drive wheel and the track, yet will not elevate the center of mass of the vehicle. Preferably, an apparatus is needed that will allow the drive wheel to be elevated for better friction, but will not require that the differential be raised as well. The present invention provides such an apparatus.